1. Field of the Invention
The present invention relates generally to the erasure of magnetically recorded data. In particular, the present invention relates to AC erase current circuits for erasing digital data recorded on magnetic media such as magnetic tapes and disks.
2. Description of the Prior Art
Data is conventionally recorded on magnetic media by varying the magnetization along data tracks on the surface of the media. For example, digital data may be recorded at a particular point along a data track by magnetizing the surface of the media to provide a detectable flux transition at the desired point to represent a digital one or no flux transition at that point to represent a digital zero. The location of the point of interest is typically determined with an external reference, such as a clock pulse or magnetically recorded servo data.
By its nature magnetic media is typically written many times, so that when data is recorded, it must be assumed that the data to be recorded is to be written over previously recorded data. In order to provide accurate and reproducible results, the recording processes therefore often includes an erasing step in which previously recorded data, if any, is erased before the new data is recorded. A read after write step is then often performed in which the newly written data is then immediately read to verify that it was accurately written.
Erasure is conventionally achieved by demagnetizing the media to remove all flux transitions. Erasure is typically performed by an electromagnet, known as a magnetic head, placed in the vicinity of the surface of the magnetic media and driven by an electrical current to form a magnetic field at the appropriate time to erase the data. Early erasure heads used direct current or DC excitation which resulted in a magnetic field having a constant magnitude and direction so that the flux transitions representing recording data, if any, were effectively obliterated.
Improved erasure characteristics were provided by the use of alternating current or AC excitation of the erasure head. An AC excited erase head applies a rapidly alternating magnetic field to the media. As distinct from the DC excited erase head which left the media with magnetic saturation at one level, the AC excited erase head subjects the magnetic media to a rapidly fluctuating field which is reduced in magnitude as the relative position between the media and erase head is increased by, for example, tape motion. The rapidly fluctuating, decreasing field results in minimal remanent polarization of the magnetic media, reducing noise and improving the quality of the later written and recorded data.
Conventional AC excited erase head designs, one of which is shown for example, in U.S. Pat. No. 4,666,027 issued in 1984 to Howell, Miller and Buchan and assigned to Archive Corporation, are relatively complex and inefficient. Since the magnitude of the applied erase field typically varies in accordance with component selection, component selection becomes critical. In accordance with conventional manufacturing processing, the actual value of the erase head inductance can vary as much as 20% from the design value; the actual value of the capacitors can vary by as much as 10% from the design value. Consequently, the frequency of the excitation may generally not match the resonant frequency of the tank. This problem has typically been addressed by costly individual tuning of each erase head, by using an erase head of such high efficiency that the lowest erase current will be sufficient to perform adequate erase, or by driving the erase head hard enough so that the lowest resulting field magnitude will be sufficient to provide satisfactory erasure.
Additionally, conventional AC excited erase heads produce unwanted harmonic oscillations at multiples of their excitation or resonant frequencies. These unwanted higher frequency signals interfere with other operations of the data recording and retrieval systems by, for example, interfering with the read after write step described above. Conventional AC excited erase heads therefore typically require harmonic suppression circuitry, typically in the power supply circuitry, to minimize interference from such unwanted signals.
What is needed is a circuit which further improves the quality and consistent magnitude of the erase field, and in particular, a circuit which reduces the need for costly individual tuning and critical component selection.